Flame detection is the technology for detecting flames, using a flame detector. Flame detectors are optical equipment for the detection of flame phenomena of a fire. There are two categories of flame detection:

An ultraviolet (UV) detector responds to radiation in the spectral range of approximately 180 to 260 nm. This frequency range is the least sensitive for natural background radiation sources like cosmic radiation and especially sunlight. The sunlight is, in the higher frequencies, absorbed by almost all vapours and gases; especially by ozone and smoke but also by an oil or grease film on the window of a flame detector. Almost every fire radiates UV light, and the UV sensor is a good all around flame detector. A disadvantage is that quite a few artificial false-alarm sources occur; like halogen and quartz lighting (without regular glass), electrical welding, corona and static arcs.

A visible light sensor (for example a camera: 0.4 to 0.7 µm) is able to present an image, which can be understood by a human being. Furthermore complex image processing analysis can be executed by computers, which can recognize a flame or even smoke. Unfortunately, a camera can be blinded, like a human, by heavy smoke and by fog. It is also possible to mix visible light information (monitor) with UV or Infrared information, which in this case is made visible. The corona camera is an example of this equipment. In this equipment the information of an UV camera mixed with visible image information. It is used for tracing defects in high voltage equipment and fire detection over high distances.

A near Infrared(IR) sensor (0.7 to 1.1 µm) is especially able to monitor flame phenomena, without too much hindrance from water and water vapour. Pyroelectric sensors operating at this wavelength can be relatively cheap. Multiple channel or pixel array sensors monitoring flames in the near IR band are arguably the most reliable technologies available for detection of fires. Light emission from a fire forms an image of the flame at a particular instant. Digital image processing can be utilized to recognize flames through analysis of the video created from the near IR images.

A wideband infrared sensor (1.1 µm and higher) monitors especially the heat radiation of a fire. A special frequency range is 4.3 to 4.4 µm. This is a resonance frequency of CO2. During burning of a hydrocarbon (for example, wood or fossil fuels such as oil and natural gas) much heat and CO2 is released. The hot CO2 emits much energy at its resonance frequency of 4.3 µm. This causes a peak in the total radiation emission and can be well detected. Moreover, the "cold" CO2 in the air is taking care that the sunlight and other IR radiation is filtered. This makes the sensor in this frequency "Solar blind", however sensitivity is reduced by sunlight. By observing the flicker frequency of a fire (1 to 20 Hz) the detector is made less sensitive to false alarms caused by heat radiation, for example caused by hot machinery. Multi-Infrared detectors make use of algorithms to suppress the effects of background radiation (blackbody radiation), again sensitivity is reduced by this radiation.

A severe disadvantage is that almost all radiation can be absorbed by water or water vapour; this is particularly valid for infrared flame detection in the 4.3 to 4.4 µm region. From approx. 3.5 µm and higher the absorption by water or ice is practically 100%. This makes infrared sensors for use in outdoor applications very unresponsive to fires. The biggest problem is our ignorance, some infrared detectors have an (automatic) detector window self test, but this self test only monitors the occurrence of water or ice on the detector window.

A salt film is also harmful, because salt absorbs water. However, water vapour, fog or light rain also makes the sensor almost blind, without the user knowing. The cause is similar to what a fire fighter does if he approaches a hot fire: he protects himself by means of a water vapour screen against the enormous infrared heat radiation. The presence of water vapor, fog, or light rain will then also "protect" the monitor causing it to not see the fire. Visible light will, however be transmitted through the water vapour screen, as can easily been seen by the fact that a human can still see the flames through the water vapour screen.

A fire emits radiation, which human eye experiences as the visible yellow red flames and heat. In fact, during a fire, relatively sparsely UV energy and visible light energy is emitted, as compared to the emission of Infrared radiation. A non-hydrocarbon fire, for example, one from hydrogen, does not show a CO2 peak on 4.3 µm because during the burning of hydrogen no CO2 is released. The 4.3 µm CO2 peak in the picture is exaggerated, and is in reality less than 2% of the total energy of the fire. A multi-frequency-detector with sensors for UV, visible light, near IR and/or wideband IR thus have much more "sensor data" to calculate with and therefore are able to detect more types of fires and to detect these types of fires better: hydrogen, methanol, ether or sulphur. It looks like a static picture, but in reality the energy fluctuates, or flickers. This flickering is caused by the fact that the aspirated oxygen and the present combustible are burning and concurrently aspirate new oxygen and new combustible material. These little explosions cause the flickering of the flame.

The sun emits an enormous amount of energy, which would be harmful to human beings if not for the vapours and gases in the atmosphere, like water (clouds), ozone, and others, through which the sunlight is filtered. In the figure it can clearly be seen that "cold" CO2 filters the solar radiation around 4.3 µm. An Infrared detector which uses this frequency is therefore solar blind. Not all manufacturers of flame detectors use sharp filters for the 4.3 µm radiation and thus still pick up quite an amount of sunlight. These cheap flame detectors are hardly usable for outdoor applications. Between 0.7 µm and approx. 3 µm there is relatively large absorption of sunlight. Hence, this frequency range is used for flame detection by a few flame detector manufacturers (in combination with other sensors like ultraviolet, visible light, or near infrared). The big economical advantage is that detector windows can be made of quartz instead of expensive sapphire. These electro-optical sensor combinations also enable the detection of non-hydrocarbons like hydrogen fires without the risk of false alarms caused by artificial light or electrical welding.

Infrared flame detectors suffer from Infrared heat radiation which is not emitted by the possible fire. One could say that the fire can be masked by other heat sources. All objects which have a temperature higher than the absolute minimum temperature (0 kelvins or −273.15 °C) emit energy and at room temperature (300 K) this heat is already a problem for the infrared flame detectors with the highest sensitivity. Sometimes a moving hand is sufficient to trigger an IR flame detector. At 700 K a hot object (black body) starts to emit visible light (glow). Dual- or multi-infrared detectors suppress the effects of heat radiation by means of sensors which detect just off the CO2 peak; for example at 4.1 µm. Here it is necessary that there is a large difference in output between the applied sensors (for example sensor S1 and S2 in the picture). A disadvantage is that the radiation energy of a possible fire must be much bigger than the present background heat radiation. In other words, the flame detector becomes less sensitive. Every multi infrared flame detector is negatively influenced by this effect, regardless how expensive it is.

The cone of vision of a flame detector is determined by the shape and size of the window and the housing and the location of the sensor in the housing. For infrared sensors also the lamination of the sensor material plays a part; it limits the cone of vision of the flame detector. A wide cone of vision does not automatically mean that the flame detector is better. For some applications the flame detector needs to be aligned precisely to take care that it does not detect potential background radiation sources. The cone of vision of the flame detector is three dimensional and is not necessarily perfectly round. The horizontal angle of vision and the vertical angle of vision often differ; this is mostly caused by the shape of the housing and by mirroring parts (meant for the self test). Different combustibles can even have a different angle of vision in the same flame detector. Very important is the sensitivity at angles of 45°. Here at least 50% of the maximum sensitivity at the central axis must be achieved. Some flame detectors here achieve 70% or more. In fact these flame detectors have a total horizontal angle of vision of more than 90°, but most of the manufacturers do not mention this. A high sensitivity on the edges of the angle of vision provides advantages for the projection of a flame detector.

The range of a flame detector is highly determined by the mounting location. In fact, when making a projection, one should imagine in what the flame detector “sees”. A rule of thumb is, that the mounting height of the flame detector is twice as high as the highest object in the field of view. Also the accessibility of the flame detector must be taken into account, because of maintenance and/or repairs. A rigid light-mast with a pivot point is for this reason recommendable. A “roof” on top of the flame detector (30 x 30 cm, 1 x 1-foot) prevents quick pollution in outdoor applications. Also the shadow effect must be considered. The shadow effect can be minimized by mounting a second flame detector in the opposite of the first detector. A second advantage of this approach is, that the second flame detector is a redundant one, in case the first one is not working or is blinded. In general, when mounting several flame detectors, one should let them “look” to each other not let them look to the walls. Following this procedure blind spots (caused by the shadow effect) can be avoided and a better redundancy can be achieved than if the flame detectors would “look” from the central position into the to be protected area. The range of flame detectors to the 30 x 30 cm, 1 x 1-foot industry standard fire is stated within the manufacturers data sheets and manuals, this range can be affected by the previously stated de-sensitizing effects of sunlight, water, fog, steam and blackbody radiation.

If the distance between the flame and the flame detector is large compared to the dimension of the fire then the square law applies: If a flame detector can detect a fire with an area A on a certain distance, then a 4 times bigger flame area is necessary if the distance between the flame detector and the fire is doubled. In short:

This law is equally valid for all optical flame detectors, including video based ones. The maximum sensitivity can be estimated by dividing the maximum flame area A by the square of the distance between the fire and the flame detector: c = A/d2. With this constant c can, for the same flame detector and the same type of fire, the maximum distance or the minimum fire area be calculated: A = cd2 and d = √(A/c). It must be emphasized, however, that the square root in reality is not valid anymore at very high distances. At long distances other parameters are playing a significant part; like the occurrence of water vapour and of cold CO2 in the air. In the case of a very small flame, on the other hand, the decreasing flickering of the flame will play an increasing part.

A more exact relation - valid when the distance between the flame and the flame detector is small - between the radiation density, E, at the detector and the distance, D, between the detector and a flame of effective radius, R, emitting energy density, M, is given by